Nb fit two of my four. I would be safe and take 0-3. My last ds all we had to take him home in was nb and while it was cute that he was kinda popping all the buttons out, I wished I'd thought to ask dh to bring something bigger. Or just pack one of each size. Unless you push out a 10+ pounder I would say it would be very unlikely 3-6mos would fit. Grandma has to remember that the clothing makers (and babies) have changed tremendously in the last 30 years. There was a time where a 5lb baby was considered "average".